KVM: arm64: Consume pending SError as early as possible

On systems with v8.2 we switch the 'vaxorcism' of guest SError with an
alternative sequence that uses the ESB-instruction, then reads DISR_EL1.
This saves the unmasking and remasking of asynchronous exceptions.

We do this after we've saved the guest registers and restored the
host's. Any SError that becomes pending due to this will be accounted
to the guest, when it actually occurred during host-execution.

Move the ESB-instruction as early as possible. Any guest SError
will become pending due to this ESB-instruction and then consumed to
DISR_EL1 before the host touches anything.

This lets us account for host/guest SError precisely on the guest
exit exception boundary.

Because the ESB-instruction now lands in the preamble section of
the vectors, we need to add it to the unpatched indirect vectors
too, and to any sequence that may be patched in over the top.

The ESB-instruction always lives in the head of the vectors,
to be before any memory write. Whereas the register-store always
lives in the tail.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/include/asm/kvm_asm.h

@@ -34,7 +34,7 @@
  * Size of the HYP vectors preamble. kvm_patch_vector_branch() generates code
  * that jumps over this.
  */
-#define KVM_VECTOR_PREAMBLE	(1 * AARCH64_INSN_SIZE)
+#define KVM_VECTOR_PREAMBLE	(2 * AARCH64_INSN_SIZE)
 
 #ifndef __ASSEMBLY__
 

arch/arm64/kvm/hyp/entry.S

@@ -127,8 +127,8 @@ ENTRY(__guest_exit)
 
 alternative_if ARM64_HAS_RAS_EXTN
 	// If we have the RAS extensions we can consume a pending error
-	// without an unmask-SError and isb.
-	esb
+	// without an unmask-SError and isb. The ESB-instruction consumed any
+	// pending guest error when we took the exception from the guest.
 	mrs_s	x2, SYS_DISR_EL1
 	str	x2, [x1, #(VCPU_FAULT_DISR - VCPU_CONTEXT)]
 	cbz	x2, 1f
@@ -146,7 +146,6 @@ alternative_else
 	mov	x5, x0
 
 	dsb	sy		// Synchronize against in-flight ld/st
-	nop
 	msr	daifclr, #4	// Unmask aborts
 alternative_endif
 

arch/arm64/kvm/hyp/hyp-entry.S

@@ -226,6 +226,7 @@ ENDPROC(\label)
 .macro valid_vect target
 	.align 7
 661:
+	esb
 	stp	x0, x1, [sp, #-16]!
 662:
 	b	\target
@@ -237,6 +238,7 @@ check_preamble_length 661b, 662b
 	.align 7
 661:
 	b	\target
+	nop
 662:
 	ldp	x0, x1, [sp], #16
 	b	\target
@@ -269,7 +271,8 @@ ENDPROC(__kvm_hyp_vector)
 #ifdef CONFIG_KVM_INDIRECT_VECTORS
 .macro hyp_ventry
 	.align 7
-1:	.rept 27
+1:	esb
+	.rept 26
 	nop
 	.endr
 /*
@@ -317,6 +320,7 @@ ENTRY(__bp_harden_hyp_vecs_end)
 	.popsection
 
 ENTRY(__smccc_workaround_1_smc_start)
+	esb
 	sub	sp, sp, #(8 * 4)
 	stp	x2, x3, [sp, #(8 * 0)]
 	stp	x0, x1, [sp, #(8 * 2)]